It is known that isobutene and other isoalkenes (tertiary olefins) produced by hydrocarbon cracking may be reacted with methanol or other C.sub.1 -C.sub.3 lower aliphatic alcohols over an acidic catalyst to provide tertiary butyl ether, especially MTBE and TAME. Generally, it is known that asymmetrical ethers having the formula (CH.sub.3).sub.3 C-O-R, where R is a C.sub.1 -C.sub.4 alkyl radical, are particularly useful as octane improvers for liquid fuels, especially gasoline.
MTBE, ethyl t-butyl ether (ETBE), TAME and isopropyl t-butyl ether (IPTBE) are known to be high octane ethers. The article by J.D. Chase, et al., Oil and Gas Journal, April 9, 1979, discusses the advantages one can achieve by using such materials to enhance gasoline octane. The octane blending number of MTBE when 10% is added to a base fuel (R+O=91) is about 120. For a fuel with a low motor rating (M+O=83) octane, the blending value of MTBE at the 10% level is about 103. On the other hand, for an (R+O) of 95 octane fuel, the blending value of 10% MTBE is about 114.
Liquid phase reaction of methanol with isobutylene and isoamylenes at moderate conditions with a resin catalyst is known technology, as provided by R. W. Reynolds, et al., The Oil and Gas Journal, June 16, 1975, and S. Pecci and T. Floris, Hydrocarbon Processing, December 1977. An article entitled "MTBE and TAME--A Good Octane Boosting Combo," by J.D. Chase, et al., The Oil and Gas Journal, April 9, 1979, pages 149-152, discusses the technology. Preferred catalysts are polymeric sulfonic acid exchange resin such as Amberlyst 15 and zeolites such as zeolite Beta and ZSM-5. The acid resin catalysts are effective catalysts at temperatures below 90.degree. C. At higher temperatures the resin catalyst is unstable. Typically, with acid resin catalyst the etherification reaction is carried out in liquid phase. However, vapor phase and mixed phase etherification is known, particularly where the catalyst is contained as a fixed bed in a catstill type fractionator which serves to both separate the reaction products and operate as a vessel to contain the catalyst under etherification conditions.
Typical hydrocarbon feedstock materials for etherification reactions include olefinic streams, such as cracking process light gas containing butene isomers in mixture with substantial amounts of paraffins including n-butane and isobutane. The C.sub.4 components usually contain a major amount of unsaturated compounds, such as 10-40% isobutene, 20-55% linear butenes, and small amounts of butadiene. Also, C.sub.4 + heavier olefinic hydrocarbon streams may be used, particularly mixtures of isobutene and isoamylene and C.sub.5 + streams containing isoamylene.
Isoalkanes such as isobutane and isopentanes can be dehydrogenated to isoalkenes or tertiary olefins such as isobutene and isoamylenes and etherified with methanol to provide MTBE and TAME.
Dehydrogenation of isoalkanes in the vapor phase is well known in the art to produce isoalkenes. When in the prior art dehydrogenation is integrated with etherification to produce MTBE and TAME, isoalkene vapor from dehydrogenation is first condensed to employ as a feedstream in the conventional liquid phase etherification of iso-olefins of tertiary alkyl ethers. Following etherification, unconverted iso-olefins is the etherification reactor effluent are vaporized to recycle them to the dehydrogenation reactor. Prior integrated dehydrogenation-etherification process require at least two phase changes with significant energy losses and inefficiencies associated with these changes as well as the need for multiple heat exchangers.
U.S. Pat. No. 4,605,787 to Chu et al., incorporated herein by reference, describes a process for the preparation of methyl tertiary butyl ether which comprises reacting isobutene and methanol in the vapor phase in the presence of zeolite catalyst. Under the conditions described for the vapor phase etherification, side reactions, particularly the dimerization of isobutene, are virtually eliminated. The reaction products are essentially MTBE and unreacted methanol and/or isobutene.
U.S. Pat. No. 5,008,467 to Vora et al. describes a process for direct etherification of a dehydrogenation effluent using liquid phase etherification. Light components are not purged in a membrane system; nor is a single tower used to debutanize the MTBE product and recover MTBE product. Conventional practices are followed as well with respect to the provision of a reflux stream for the fractionator employed in the process of Vora et al.
It is an object of the present invention to provide an improved process for the production of alkyl tertiary butyl ether or alkyl tertiary amyl ether from isobutane and/or isopentane.
A further object of the invention is to provide a substantially vapor phase integrated process for dehydrogenation of isoalkanes and etherification of resultant isoalkenes to tertiary alkyl ethers.
Another object of the invention is to improve the overall energy efficiency of converting isoalkanes to alkyl tertiary alkyl ethers by avoiding the need to condense at least a major portion of the effluent from dehydrogenation of isoalkane and revaporize the effluent from isoalkene etherification so that unreacted isoalkene or isoalkane can be recycled to the dehydrogenation vessel.